This invention relates to a process for the preparation of N-phosphonomethylglycine, and more particularly to the preparation of N-phosphonomethylglycine by the conversion of N-phosphonomethyliminodiacetic acid to N-phosphonomethylglycine using peroxides.
N-Phosphonomethylglycine, known also by its common name glyphosate, is a highly effective commercially important phytotoxicant useful in controlling a large variety of weeds. It is applied to the foliage of a very broad spectrum of annual and perennial grasses and broad leaf plants. Industrial uses include control of weeds along roadsides, waterways, transmission lines, in storage areas, and in other nonagricultural areas. Usually, N-phosphonomethylglycine is formulated into herbicidal compositions in the form of its various salts in solution, preferably water.
U.S. Pat. No. 3,950,402 to Franz discloses a process for the production of N-phosphonomethylglycine by forming an admixture of N-phosphonomethyliminodiacetic acid, water, and a metallic catalyst selected from the noble metals, heating the admixture to an elevated temperature (greater than 70.degree. C. to avoid low yields) and contacting the admixture with a free oxygen-containing gas.
U.S. Pat. No. 3,954,848 to Franz discloses a process for the production of N-phosphonomethylglycine by reacting N-phosphonomethyliminodiacetic acid with an oxidizing agent, such as hydrogen peroxide, in an aqueous acidic medium in the presence of a strong acid at a temperature of from about 70.degree. C. to about 100.degree. C. It is disclosed that one should employ at least 2 moles of the hydrogen peroxide for each mole of the N-phosphonomethyliminodiacetic acid, and preferably more.
Hungarian Patent Application No. 187,347 discloses a process for the preparation of N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid with peroxides using a catalytic amount of a metal compound selected from compounds of silver, iron, tin, lead, manganese, or molybdenum. Molybdates are preferred. At temperatures lower than 80.degree. C., usually a contaminated end-product is obtained. Typically, the reaction is carried out at a temperature of above 80.degree. C. and preferably above 100.degree. C. at pressures exceeding atmospheric.
Although satisfactory results are obtained by the above processes to make N-phosphonomethylglycine, all of them suffer from one or more disadvantages, such as the use of excessive amounts of peroxide, the use of strong mineral acids and/or reaction at elevated temperatures and pressures. Thus, there is a need for a process which provides N-phosphonomethylglycine in high yields at modest temperatures and at atmospheric pressure using substantially stoichiometric amounts of peroxide to oxidize the N-phosphonomethyliminodiacetic acid to the desired N-phosphonomethylglycine without using strong mineral acids such as hydrochloric acid or sulfuric acid.